Effects of Stress and Electromigration on Microstructural Evolution in Microbumps of Three-Dimensional Integrated Circuits

Due to geometric scaling, the heterogeneous and anisotropic microstructures present in through-silicon vias and microbumps must be considered in the stress management of 3-D integrated circuits. In this paper, a phase field model is developed to investigate the effects of stress and electromigration on microstructural evolution in a Cu/Sn-microbump/Cu structure at 150 °C. External compressive stress is observed to accelerate the growth of Cu₃Sn grains and cause the separation of continuous interfacial Cu₆Sn₅ grains by β-Sn grains, whereas tensile stress promotes the growth of Cu₆Sn₅ grains and the formation of a continuous Cu₆Sn₅ layer. The roughness of the β-Sn-Cu₆Sn₅ interface under compressive stress is greater than that under tensile stress. The morphological evolution of the β-Sn grains is also affected by stress. An external shear or compressive stress favors the growth of the β-Sn grains with their c-axis particular to the V-direction. Furthermore, the interdiffusion flux driven by electromigration increases the roughness of the interfacial Cu₆Sn₅ grains at the cathode. The strain caused by electromigration results in larger β-Sn grains, enabling faster interdiffusion along the current direction. The preferential growth of the β-Sn grains under stress or electromigration decreases the shear modulus of microbumps.